Process for the manufacture of a component for bearings and its products

ABSTRACT

Steel for the manufacture of a component for bearings, the chemical composition of which comprises, by weight: 0.6%≦C≦1.5%; 0.4%≦Mn≦1.5%; 0.75%≦Si≦2.5%; 0.2%≦Cr≦2%; 0%≦Ni≦0.5%; 0%≦Mo≦0.2%; 0%&lt;Al≦0.05%; S≦0.04%; the balance being iron and impurities resulting from smelting and the composition furthermore satisfying the relationships: Mn≦0.75+0.55×Si and Mn≦2.5−0.8×Si. Process for the manufacture of a bearing component.

This application is a Division of application Ser. No. 09/055,357 Filedon Apr. 6, 1998, now U.S. Pat. No. 6,162,390 issued Dec. 19, 2000.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to steel particularly suited for bothgeneral use and the manufacture of a component for bearings, especiallya race for ball bearings, needle bearings or roller bearings.

BACKGROUND OF THE INVENTION

Components for bearings, such as races, balls, needles or rollers, aregenerally manufactured from steel of the 100Cr6 or 100CrMn6 typecontaining from 0.6 to 1.5% of carbon, from 1.3 to 1.6% of chromium,from 0.3 to 1% of manganese and less than 0.4% of silicon and having avery high degree of cleanliness in terms of inclusions. The steel isused in the form of rolled bar, seamless tube or wire, from which arecut blanks or slugs which are formed by cold or hot plastic deformationand then hardened by tempering and annealing, before being machined. Thecomponents thus obtained have a high hardness and the toughness requiredfor them to be able to withstand the rolling fatigue well, at leastunder the normal conditions of use, especially for in-servicetemperatures below 150° C. However, the components thus formed have aninsufficient rolling fatigue resistance for more severe serviceconditions, which are tending to become common. These more severeservice conditions are characterized, in particular, by a servicetemperature above 150° C. and possibly as high as 350° C., and/or by thepresence of the phenomenon of bearing surface deterioration byindentation. This phenomenon consists of the initiation of cracks on thesurface, caused by the indentations, i.e. deformations generated by hardparticles present in the lubricant.

In order to limit the effect of the indentation, it has been proposed touse materials having a very high hardness such as ceramics or depositsof hard materials. However, this technique has the drawback of being notvery reliable because of the excessively high brittleness of thesematerials, which brittleness makes them very sensitive to the slightestdefect.

It has also been proposed, for example in U.S. Pat. No. 5,030,017, touse a steel containing, in particular, from 0.3% to 0.6% of carbon, from3% to 14% of chromium, from 0.4% to 2% of molybdenum, from 0.3% to 1% ofvanadium and less than 2% of manganese. The components are carburized orcarbonitrided in the region of the bearing surface, so as to obtain asum of the carbon and nitrogen contents of between 0.03% and 1%, and arethen hardened so that their micrographic structure comprises from 20% to50% (in % by volume) of residual austenite in a surface layerrepresenting from 10% to 25% of the volume of the component. Thistechnique has the double drawback of requiring the use of a steel whichis highly loaded with alloying elements, and is hence expensive, and theexecution of a carburizing or carbonitriding treatment, this treatmentbeing lengthy and expensive.

It has also been proposed, in German Patent Application DE 195 24 957,to use a steel containing from 0.9% to 1.3% of carbon, from 0.6% to 1.2%of silicon, from 1.1% to 1.6% of manganese and from 1.3% to 1.7% ofchromium, the balance being iron and impurities resulting from smelting,the structure of this steel containing from 7% to 25% of residualaustenite. However, this steel, because of its chemical composition,provides no guarantee of castability, of cold deformability and ofresidual austenite content and stability. The specified residualaustenite content necessary for improving the resistance to indentationfatigue moreover requires, in the case of this steel, subjecting thebearings to a not very convenient heat treatment comprising a step ofholding them at approximately 100° C. for more than 10 hours betweentempering and annealing without returning to ambient temperature aftertempering or before annealing. Moreover, in the presence ofmultidirectional stresses below the cyclic yield stress, its austeniteis stable for more than 2000 hours only for thermal stresses below 120°C., which is too low for some applications.

SUMMARY OF THE INVENTION

The object of the present invention is to remedy these drawbacks byproviding a means for manufacturing, in an economical manner andespecially using a relatively standard heat treatment, a component forbearings which is resistant to indentation, in particular when they areused briefly or occasionally above 300° C., and is not very brittle.

For this purpose, the subject of the invention is a steel usefulgenerally and for the manufacture of a component for bearings, thechemical composition of which comprises, by weight:

0.6%≦C≦1.5%

0.4%≦Mn≦1.5%

0.75%≦Si≦2.5%

0.2%≦Cr≦2%

0%≦Ni≦0.5%

0%≦Mo≦0.2%

0%≦Al≦0.05%

0%≦Ti≦0.04%

S≦0.04%

the balance being iron and impurities resulting from smelting, thecomposition furthermore satisfying the relationships:

Mn≦0.75+0.55×Si

Mn≦2.5−0.8×Si

Preferably, the chemical composition of the steel is such that,separately or better still at the same time, on the one hand:

0.8%≦Mn≦1.2%

0.8%≦Si≦1.7%

and, on the other hand:

0.9%≦C≦1.1%

1.3%≦Cr≦1.6%

Also preferably, the silicon content is greater than 1.2%; the inventorshave, in fact, surprisingly observed that, when simultaneously thesilicon content is greater than 1.2% and the manganese content is lessthan 1.5%, and preferably less than 1.2% but greater than 0.8%, thestability of the austenite is very substantially improved.

The invention also relates to a process for the manufacture of acomponent for bearings, in which:

a semi-finished product made of steel according to the invention isprovided;

the semi-finished product is formed by hot plastic deformation so as toobtain a product blank and, more particularly, a seamless-tube blank

a spheroidizing treatment is carried out on the product blank, thistreatment comprising heating to a temperature of between 750° C. and850° C. followed by cooling, the maximum rate of which is 10° C./hour,down to 650° C. so as to obtain a structure having a hardness of lessthan 270 HV and comprising a fine dispersion of carbides, andoptionally, forming by cold plastic deformation, for example coldrolling or cold drawing so as to obtain a product;

the product is cut in order to obtain a section which is formed by coldor hot plastic deformation, or by machining, so as to obtain a componentblank for bearings; and

the component blank for bearings is subjected to a hardening heattreatment by cooling, for example in oil, after austenization between800° C. and 950° C. and to an annealing heat treatment between 100° C.and 400° C. and preferably below 250° C., so as to obtain a componentfor bearings which has a structure whose hardness is between 58 HRC and67 HRC and which consists of residual carbides, martensite and from 5%to 30% of residual austenite.

Finally, the invention relates, on the one hand, to a seamless tube madeof steel according to the invention and, on the other hand, to acomponent for bearings made of steel according to the invention having astructure consisting of residual carbides, martensite and from 5% to 30%of residual austenite thermally stable up to 400° C. at least.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described in greater detail, but in anon-limiting manner, and illustrated by examples.

In order to manufacture a bearing component, such as a race or a rollingbody having a good resistance to indentation, for example from aseamless tube, a steel is used whose chemical composition comprises, byweight:

more than 0.6% and preferably more than 0.9% of carbon in order toobtain a sufficient hardness and a sufficient degree of residualaustenite, but less than 1.5% and preferably less than 1.1% in order toavoid the formation of excessive segregation and to limit the formationof primary carbides;

more than 0.75% and preferably more than 0.8%, better still more than1.2% silicon so as to increase the hot stability of the residualaustenite (between 170° C. and 450° C. approximately and preferablyabove 300° C.) and the hardness, but less than 2.5% and preferably lessthan 1.7% since, when the silicon content is too high, the steel becomestoo brittle, especially for being able to be formed by plasticdeformation;

more than 0.4% of manganese and preferably more than 0.8% in order to beable to obtain a hardened structure having a residual austenite contentgreater than 5% and preferably greater than 15%; the manganese contentmust be such that: Mn≦0.75+0.55×Si in order to obtain good castabilitywithout which it becomes difficult to obtain a sufficiently clean steelso that it has a good resistance to rolling fatigue, and such that:Mn≦2.5−0.8×Si in order to allow finishing operations and forming by coldplastic deformation; it follows from these relationships that themanganese content must be less than 1.5% and it is preferable for it tobe less than 1.2%;

from 0.2% to 2% of chromium, and preferably from 1.3% to 1.6%, so as, onthe one hand, to obtain sufficient hardenability and, on the other hand,to form seeds of spheroidal carbides with a size of less than 2 μm whichare uniformly distributed and in sufficient quantity;

less than 0.5% of nickel, which residual element is not absolutelyessential but which has a favorable effect on the hardenability;

less than 0.2% of molybdenum, which element slows down the rate ofsoftening during annealing;

between 0% and 0.05% of aluminium and less than 0.04% of sulfur, thebalance being iron and impurities resulting from smelting.

This steel is cast and, optionally, rolled in order to manufacture asemi-finished product which, when it is desired to manufacture a racefrom a seamless tube, is a tube round.

The semi-finished product is then formed by hot plastic deformation inorder to obtain a product blank, for example by hot rolling in order toobtain a seamless tube.

The product blank is then subjected to a spheroidizing heat treatmentconsisting of heating to a temperature of between 750° C. and 850° C.followed by cooling, the maximum rate of which is 10° C./hour, down to650° C. so as to obtain a structure having a hardness of less than 270HV and comprising a fine dispersion of carbides. This heat treatment isnecessary so that the steel has a good capability of being formed bycold plastic deformation and a good machinability, the process used tomanufacture a product, for example by cold rolling or by cold drawing.This product, which may be a seamless tube, is characterized in that itis well gaged. It is used to manufacture the blanks for components, forexample the blanks for bearing races.

The manufacture of the component blanks, which is carried out by thecold or hot forming or machining of sections cut from the product, endsin a heat treatment consisting of hardening by tempering and annealing.A component for bearings is thus obtained. The prehardeningaustenization temperature, greater than 800° C., is adjusted so as toobtain, after hardening, a structure consisting of martensite, from 5%to 30% of residual austenite and a network of residual carbides. Thedegree of residual austenite, the presence of which is essential forobtaining good resistance to indentation, depends on the value of the MSpoint (martensitic transformation start temperature) which itselfdepends both on the composition of the steel and on the austenizationconditions. The person skilled in the art knows how to determine theseparameters, for example using dilatometric tests. The annealing, whichis more exactly a stress relieving, is carried out by heating above 100°C. so as to stabilize the structure, but below 400° C. and preferablybelow 250° C. in order to destabilize the residual austenite.

By way of a first example, 10 laboratory castings are produced, 2according to the invention (labelled A and B) and 8 by way of comparison(labelled C, D, E, F, G, H, I and J). These castings, to which thestandard 100Cr6 was added, which are essentially intended to demonstratethe effect of the alloying elements on the various properties of abearing steel, had the following chemical compositions (in % by weight;only the main elements are indicated, the balance being iron andimpurities):

C Si Mn Ni Cr Mo Al S A 0.957 1.508 1.006 0.138 1.632 0.019 0.033 0.008B 0.972 1.080 1.100 0.161 1.520 0.023 0.038 0.010 C 0.950 2.501 1.0160.132 1.571 0.021 0.034 0.007 D 0.959 2.508 2.074 0.126 1.607 0.0210.033 0.007 E 0.938 0.446 2.110 0.129 1.605 0.020 0.035 0.008 F 0.9721.509 2.045 0.124 1.539 0.019 0.032 0.008 G 0.950 1.513 0.263 0.1311.570 0.020 0.027 0.006 H 0.952 0.501 1.022 0.139 1.606 0.021 0.0220.004 I 0.985 1.040 0.345 0.149 1.490 0.017 0.032 0.009 J 0.966 2.0600.297 0.159 1.520 0.019 0.038 0.006 100Cr6 1.000 0.200 0.300 — 1.500 — ——

These steels were cast in the form of 65 kg ingots which were forged inorder to form square bars of side 20 mm and then spheroidized by holdingthem for 1 hour at 30° C. above the pearlite-to-austenite transformationfinish temperature followed by cooling down to 650° C. at a rate ofbetween 8 and 10° C. per hour and completed by cooling in air down toambient temperature. The cold deformability has, in this case, beenevaluated by measuring the toughness KCU at 60° C. expressed in daJ/cm²;when this toughness is greater than 4.2 daJ/cm², the cold formability isgood and is poor in the opposite case. The ingots were subsequentlyhardened in cold oil, after austenization at 895° C., and, on the onehand, the degree τ of residual austenite and, on the other hand, theresidual austenite destabilization start temperature θ were measured.The castability was also evaluated. The results were as follows:

KCU to 0.75 + 0.55 2.5 − 0.8 60° C. Si (%) Mn (%) Si Castability SidaJ/cm² τ θ A 1.508 1.006 1.579 good 1.294 4.2 14% 390° C. B 1.080 1.1001.344 good 1.636 5.5 17% 400° C. C 2.501 1.016 2.125 good 0.499 1.3 15%405° C. D 2.508 2.074 2.129 good 0.494 0.5 19% 440° C. E 0.446 2.1100.995 poor 2.143 5.6 nd 260° C. F 1.509 2.045 1.580 poor 1.293 2.1 22%410° C. G 1.513 0.263 1.582 good 1.290 4.6  9% 350° C. H 0.501 1.0221.025 good 2.099 6.7  9% 225° C. I 1.040 0.345 1.322 good 1.668 6.0 14%nd J 2.060 0.297 1.883 good 0.852 4.4 12% nd 100Cr6 0.200 0.300 0.86 good 2.34  6.6  7% 170° C.

These results show that only the heats A and B according to theinvention combine all the desired properties, namely good castability,good cold deformability, a high degree of residual austenite and astable structure up to high temperatures, the latter two characteristicsbeing substantially superior to the corresponding characteristics of thestandard 100Cr6.

In addition, residual austenite stability tests, under monotonic stressand in cyclic compression, showed that:

for the heats with a silicon content greater than 1%, the residualaustenite remains stable when it is subjected by compression to anequivalent shear stress of 1400 MPa while, under the same conditions,50% of the residual austenite of a 100Cr6 steel (containing less than0.5% of Si) is destabilized; and

in the case of the residual austenite destabilization tests in cycliccompression (equivalent shear stress varying. between 25 MPa and 1025MPa at a frequency of 200 Hz), no destabilization occurred after 1million cycles for a heat containing approximately 1% of manganese and1.5% of silicon (heat A).

By way of a second example, an industrial heat of a steel according tothe invention was produced, from which a race for bearings wasmanufactured. The chemical composition of the steel comprised, byweight:

C=0.9%

Si=1.25%

Mn=1%

Cr=1.4%

Ni=0.25%

Mo=0.015%

the balance being iron and impurities resulting from smelting.

This steel was cast and rolled in the form of a tube round 100 mm indiameter.

The tube round was hot pierced between two rolls and then hot rolled inorder to obtain a seamless-tube blank having an external diameter of67.5 mm and an internal diameter of 36.5 mm. The tube blank wassubjected to a spheroidizing treatment consisting of holding it for 2hours at 800° C. followed by cooling down to 650° C. at a rate of 10° C.per hour, so as to obtain a Brinell hardness of 240 HB. The tube blankwas then cold rolled in order to obtain a seamless tube having anexternal diameter of 42.9 mm and an internal diameter of 22.7 mm.

Bearing races were cut and machined from the tube and then subjected toan oil-hardening treatment after austenization at 900° C. and annealingat 200° C. so as to obtain a structure containing 18% of residualaustenite.

The indentation resistance was tested by means of tests of thebutt-fatigue type under high hertzian stress using races which werepre-indented on the tracks by two symmetrically placed Vickersindentations and by measuring the spalling times of the races. Racesaccording to the invention were thus compared with 100Cr6 racesaccording to the prior art for indentations whose diagonals measured 267μm and 304 μm. For both the races according to the invention and theraces according to the prior art the hardness was 63 HRC.

The results were as follows:

Size of the indents 267 μm 304 μm invention >269 hours 252 hours 100Cr6,prior art  145 hours  75 hours

These results show that the races according to the invention have a morethan doubled lifetime for significant indentations.

The steel according to the invention is particularly suitable for themanufacture of bearing races from seamless tubes, but it is alsosuitable for the manufacture of races, balls, rollers and needles fromrolled bar or from wire. These components may be formed by hot or coldplastic deformation, or by machining.

French Patent Application 97 04092 is incorporated herein by reference.

We claim:
 1. A process for the manufacturer of a steel component forbearings comprising: providing a semi-finished product made of steelcomprising 0.9%≦C≦1.5% 0.4≦Mn≦1.5% 0.75%≦Si≦2.5% 0.2%≦Cr≦2%  0%≦Ni≦0.5%0%≦Mo≦0.2% 0%<Al≦0.05% S≦0.04%  iron and impurities resulting fromsmelting, the composition furthermore satisfying the relationships:Mn≦0.75+0.55×Si Mn≦2.5−0.8×Si; forming the semi-finished product by hotplastic deformation so as to obtain a product blank; treating theproduct blank by a spheroidizing treatment comprising heating saidproduct blank to a temperature of between 750° C. and 850° C. followedby cooling at a maximum rate of 10° C./hour, down to 650° C. so as toobtain a structure having a hardness of less than 270 HV and comprisinga fine dispersion of carbides, and optionally, forming by cold plasticdeformation so as to obtain a product; cutting or machining a sectionfrom the product and forming said product by cold or hot plasticdeformation so as to obtain a component blank for bearings; andsubjecting the component blank to an isothermal hardening heat treatmentor cooling after austenization between 800° C. and 950° C. and to anannealing heat treatment between 100° C. and 400° C. so as to obtain acomponent for bearings which has a structure whose hardness is between58 HRC and 67 HRC and which consists of residual carbides, martensiteand from 5% to 30% of residual austenite.
 2. The process as claimed inclaim 1, wherein said isothermal hardening heat treatment is carried outat a temperature below 250° C.
 3. The process as claimed in claim 1,wherein the product blank is a seamless tube blank and wherein saidcomponent is a seamless tube.
 4. A seamless tube made of steel made bythe process of claim
 1. 5. A component for bearings, made of steel madeby the process of claim 1, having a structure consisting of a network ofcarbides, martensite and from 5% to 30% of residual austenite.
 6. Thecomponent as claimed in claim 5, which is a race.
 7. A component forbearings made by the process of claim
 1. 8. The component for bearingsof claim 7, wherein the chemical composition of said steel is such that:0.8%≦Mn≦1.2% 0.8%≦Si≦1.7%.
 9. The component for bearings of claim 7,wherein the chemical composition of said steel is such that: Si≧1.2%.10. The component for bearings of claim 7, wherein the chemicalcomposition of said steel is such that: 0.9%≦C≦1.1% 1.3%≦Cr≦1.6%. 11.The component for bearings of claim 10, wherein the chemical compositionof said steel is such that: 0.8%≦Mn≦1.2% 0.8%≦Si≦1.7%.
 12. The componentfor bearings of claim 10, wherein the chemical composition of said steelis such that: Si≧1.2%.